It comes free with udev and or openrc. It used to be /var/run but now its needed before /var is mounted, so it was migrated to /run in tmpfs.
Its some read/write space thats available very early in the boot process._________________Regards,

NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.

Oh, and must be nice to blame Poettering/RH for something that comes from Debian.

Really, debilian? I'm sorry, taking that back. But still, there's more than enough that weirdo is to blame for..._________________++++++++++[>+++++++>++++++++++>+++>+<<<<-]>++.>+.+++++++..+++.>++.<<+++++++++++++++.>.+++.------.--------.>+.>.

initramfs of course _________________"Dear Enemy: may the Lord hate you and all your kind, may you be turned orange in hue, and may your head fall off at an awkward moment."
"Linux is like a wigwam - no windows, no gates, apache inside..."

I think I'm getting old, too conventional maybe, but I really did not like it when I saw it first. Come on, you put stuff in my / without even asking? What about /tmp/run for instance (assuming /tmp is in tmpfs)? /tmp is a legit tenant in / after all._________________Please learn how to denote units correctly!

They wanted to find a place, which is not prone to be on another partition, so no /{usr,var,tmp,opt}._________________++++++++++[>+++++++>++++++++++>+++>+<<<<-]>++.>+.+++++++..+++.>++.<<+++++++++++++++.>.+++.------.--------.>+.>.

/etc is not necessarily writable. Also, this is runtime data, while /etc is for static configuration. /var is the place for such data, but they needed some writable place already before /var gets mounted.